watir测试file_field.set方法问题

ie.file_field(:xpath, "//input[@name='file1']/").set($htmlRoot + "fileupload.html")
只能弹出选择对话框，等待用户选择文件。现在想实现自动选择文件的功能，不知道目前有没有谁知道可行的解决方法
请高手指点。
附带测试脚本一份（watir提供的例子）

# feature tests for TextArea Fields
# revision: $Revision: 963 $

$LOAD_PATH.unshift File.join(File.dirname(__FILE__), '..') if $0 == __FILE__
require 'unittests/setup'

class TC_FileField_XPath < Test::Unit::TestCase
include Watir

def goto_page
$ie.goto($htmlRoot + "fileupload.html")
end

def test_file_field_Exists
goto_page
#test for existance of 4 file area
assert($ie.file_field(:xpath, "//input[@name='file1']/").exists?)
assert($ie.file_field(:xpath, "//input[@id='file2']/").exists?)
#test for missing
assert(!$ie.file_field(:xpath, "//input[@name='missing']/").exists?)
assert(!$ie.file_field(:xpath, "//input[@name='totallybogus']/").exists?)
#pop one open and put something in it.
$ie.file_field(:xpath, "//input[@name='file1']/").set($htmlRoot + "fileupload.html")
#click the upload button
$ie.button(:xpath, "//input[@name='upload']/").click

assert($ie.text.include?("PASS"))
end

def test_iterator
goto_page
assert_equal(6, $ie.file_fields.length)
end

end